Burke's Genealogical & Heraldic Guide to the Peerage, @ Christiana de Plessetis, d of Sir Hugh de Plessetis, no mother
The cited information was published by Burke's Peerage, London, 1949 (Ref: Christiana de Plessetis, d of Sir Hugh de Plessetis, no mother) The author/originator was Burke's Peerage.
Source Notes
TITL Burke's Genealogical & Heraldic Guide to the Peerage,
Baronetage & KnightageABBR Burke's Peerage